1. Field of the Invention
The invention relates to a method for determining a property of a sample from a correlogram obtainable by scanning of a surface of the sample through a focal plane of an objective while applying broad-band interferometry, and more particularly to such a method including receiving a correlogram having interference radiation intensity as a function of the scanning distance from the surface.
2. Description of the Related Art
A correlogram displaying interference radiation intensity as a function of the scanning distance from the surface may be obtainable by broadband interferometry, for example with a Mirau, Michelson and/or Linnik interferometer apparatus.
Such apparatus for determining a property (e.g. height, film thickness, refractive index) of a sample may include:
a broadband illuminator for providing a broadband illumination beam;
a beam splitter for splitting the illumination beam in a reference beam for reflection on a reference reflector and a measurement beam for reflection on a surface of the sample;
a detector for receiving an interference signal from the reflected reference beam from the reference reflector and the reflected measurement beam from the surface of the sample;
a scanning means for changing the distance between the surface of the sample and the apparatus; and
a processing means for receiving a signal representing the interference radiation intensity received on the detector from the detector and a distance signal from the scanning means and combining both to a received correlogram displaying an interference radiation intensity as a function of the scanning distance from the surface.
US Patent Publications US2007/0046953 and US2006/0262321 disclose scanning interferometry to profile a surface of an object with a complex surface structure. A cross correlation between the scanning interferometry signal and a model signal having a shape corresponding to the complex surface structure can produce a peak at the coordinate corresponding to the surface height. A disadvantage may be that information of the complex surface structure is necessary to determine the model signal corresponding to the complex surface structure. Such information may not be present for a surface of a sample.
There may be a need for an improved method for determining a property of a sample from a correlogram obtainable by scanning of the surface through a focal plane of an objective using broad-band interferometry.